As represented by bead molding technique of expandable styrene beads in a mold cavity, the expansion molding of pre-expanded pellets of a resin such as styrene resins, olefine resins, e.g. polyethylene, polypropylene, etc. or the like has hitherto been known widely. This comprises the charging of pre-expanded resin pellets into a mold cavity and meld-bonding them by steam heating followed by cooling to yield a molded product conforming to the mold contour. With this method, the charging procedure is generally conducted in such a manner that when the pre-expanded pellets are discharged from a feed-stock silo attached to a molding machine through charging equipment into the cavity of mold, the mold is not completely closed, but the mold sections are spaced apart at a clearance or gap, which is called a cracking (hereinafter the term "cracking" will be used), on the order of 0-20 mm, usually 2-8 mm and thereafter, the pre-expanded pellets are charged through the charging equipment into the mold cavity by blowing compressed air into it, thus taking advantage of suction and entrainment actions of the pre-expanded pellets.
In this case, spacing the cracking is aimed at filling the prefoamed pellets equally and homogeneously in the mold cavity by releasing the air blown therein. The mold is constructed so that even if the cracking width is larger than the diameter of the prefoamed pellets, the prefoamed pellets, when charged, ay not leak out of the mold. In spite of the fact that if the apparent density of prefoamed pellets varies, the weight of the resulting molded products will naturally vary, the method usually carried out does not take account of controlling the difference in apparent density thereby to decrease the difference in weight of the molded products. Hence, it is a common practice that the difference in weight of molded products is taken for granted. As a consequence, by taking account of the difference in weight of the molded products when produced according to the current molding method in advance, the standard in weight or expansion ratio of the products is made broad to some degree, whereby quality control (in weight) is performed.
In a situation where the weight of molded foam is measured and found to be off the standard, a worker on-site knows that the manipulation of the cracking width of the mold enables the rectification of the weight of product (expansion ratio) and he may cope with the situation on the basis of his skill by manual operation of the cracking. However, it is virtually impossible to make such adjustment frequently, even if a few times might be possible. Furthermore, in cases where prefoamed pellets disperse more than usual, the frequency of manual operation to the cracking width is increased, and consequently, in the status quo, there is practically no way of taking countermeasures.
No matter how preferable it is to enhance the quality of products by narrowing the current prevailing range of standard of product weight, the manufacturing method now in practice still has limitations as it is.
For instance, in the application of a foam material to an automobile bumper core material or the like, in which the foam serves to absorb the energy upon collision, the difference in expansion ratio of the foam directly leads to a difference in energy absorption magnitude and causes significant differences in that function with the result that excessive safeguard design against collision is necessitated. Minimizing the difference in expansion ratio of a bumper core material to the utmost is therefore desired by all means.
Also, in another example, it goes without saying that a small difference in expansion ratio of foam products is advantageous in quality or economic aspect.
In the manufacturing method of a foamed product by expansion molding of prefoamed resin pellets in a mold cavity as described above, such an approach to expansion molding has not yet been found so that the quality of products can be enhanced by making the standard range in expansion ratio of the products narrower than the current method and, nevertheless, the yield of products can be maintained at the same level as the current method. There has not been found such an expansion molding technique that, even if the apparent density of prefoamed resin pellets is more or less varied more than usual, molding is nonetheless enabled.
In view of the foregoing problems the prior art method poses, the present inventors have intensively searched for a molding method capable of solving these problems and come to the conclusion that, in the current method, weight differences of foamed products is impossible to reduce insofar as the differences in expansion ratio of prefoamed resin pellets as a feed-stock for expansion molding is not reduced. We have improved on the current method by measuring the apparent density of each feed-stock to be used, which has never been taken into account in the current methods, and obtaining each cracking width value, which governs the volume of mold charging, by the use of the resulting measurement values, thus automatically controlling the cracking width On the basis of this improvement, we have completed this invention that is capable of minimizing the weight differences of molded products.
That is, with a view toward solving the foregoing problems, the present inventors have intensively investigated into the causes as to why the expansion ratio of molded foams varies. As a result, we have found out that the prefoamed pellets as a feed-stock are difficult to expand in the same expansion ratio from pellet to pellet because of their manufactural limitations. They are a mixture of pellets having an expansion ratio distribution which is centered on a target expansion ratio and ranges on both sides of it, and even to expand the mixture strictly in an average expansion ratio of the target value is generally difficult. Accordingly, it is difficult to diminish the weight differences of molded products in the current molding method, which neglects the differences in expansion ratio of their feed-stock, insofar as the prefoamed pellets are supplied to foaming molding.
From the sampling and weighing of a definite volume of prefoamed resin pellets to be charged into a mold cavity, it has also been proved that the prefoamed pellets as stated above cause classification when delivered into a stock silo and transferred from the silo to a raw material hopper for a molding machine and, consequently, produce a wide difference in weight thereof, though it depends on the kind of prefoamed pellets.
This invention has been discovered on the basis of the foregoing findings and is aimed at making the weight of prefoamed pellets constant, thereby enabling manufacture of a molded resin foam with a minimized difference in weight.